We have isolated and characterized recombinant quail clones transformed by Rous sarcoma virus, some of which produce nonconditional defective mutants in viral replication. Recombination appears to alter the gag gene in some clones and virus is produced with new variants of the viral structural protein p19. Two nonconditional mutants were analyzed in detail. One mutant SE 21Qlb, contains a deletion near the 5' end of the genome which is necessary for RNA packaging. Another mutant, SE 52d, has an insert of quail cell information in the viral polymerase gene, and does not encode a fractional polymerase. Further characterization of these and other mutants is in progress.